ETA CEN (Eta Centauri). Centaurus, the Centaur, is filled with bright stars, the best known Rigil Kentaurus, the Alpha star, the closest to us, just 4.4 light years away. It and Hadar (Beta Cen) so dominate the constellation that the others become rather neglected, especially since the constellation is far to the south and much out of sight for much of the world's population. Few of the stars -- including Eta Cen -- even have proper names. Lying at a distance of 310 light years (give or take 25 or so), shining at second magnitude (2.31), Eta is a very hot class B (B1.5) dwarf that is part of the sprawling Scorpius-Centaurus (Sco-Cen) "OB association," really a super-association made of several smaller parts. Eta belongs to the "Upper Centaurus-Lupus" (UCL) group. Such associations are loose collections of stars born more or less at the same time and marked with blue high mass class O and B members. Not bound together by gravity, they are slowly expanding. Eta is assigned a temperature of 20,000 Kelvin, which is oddly low for its class; 24,000 should be more like it. At the lower temperature (needed to account for a lot of ultraviolet radiation), Eta would shine with the light of 5900 Suns, at the higher, 8200 (after allowing for a 15 percent reduction of light caused by interstellar dust absorption). The mass of the star thus lies between 8.5 and 10.5 solar, the radius 5 to 6 times solar, the star less than 20 million years old. Eta has several claims to special notice. First it rotates quickly with an equatorial speed of at least 310 kilometers per second, giving it a rotation period of less than a day. Second, like many of its class (Zeta Tauri for example), it is a "Be" star, one with variable hydrogen emissions in its spectrum that tell of a thick circumstellar disk that is somehow related to the rapid rotation. Eta is of an extreme sort called a "shell star," as the disk is rather edge-on to the line of sight. Third, it is a subtle, multi-periodic variable that chatters away by a few hundredths of a magnitude with periods of 18.6, 16.2, 13.5, 6.3, and 5.3 hours, all working in concert with each other. Most curious is the report of a companion star 6 seconds of arc away that was seen once in 1897 and never seen again, likely an error of some sort. The star is near the crossover between those that become massive white dwarfs and those that explode as supernovae. Only time will tell its ultimate fate.
Written by Jim Kaler. Return to STARS.